2.1 Mechanisms which Lead to Cell Death
It is widely recognized that at least two distinct cell death mechanisms exist for mammalian cells. These two mechanisms are necrosis and apoptosis, and are significant components of numerous conditions, disorders and disease states.
Necrosis plays an important physiologic role in signaling the presence of certain conditions. When cells die as a result of necrosis, the dying cells release substances that activate the body's immune response in a local, and in some cases widespread, reaction to the necrosis-inducing condition. This response is important in, for example, bacterial infection.
Experimental evidence in a wide variety of cells throughout the body has revealed that every cell can initiate a program of self-destruction, called apoptosis. This program can be initiated by a wide variety of natural and unnatural events. There are at least four distinct pathways for executing this program of cell death, and it is virtually certain that dozens, if not hundreds, of different intracellular biochemical cascades interact with each pathway. It is equally likely that certain cell types, such as cells in the heart or neurons, will utilize specialized signaling pathways that are not generally represented elsewhere in the body. However, since cell death is neither always necessary nor desired, it would be desirable to manipulate the manner in which cells start their death process. In some circumstances, preventing, delaying, or rescuing cells from death would either alleviate the disease or allow more time for definitive treatment to be administered to the patient. An example of this situation is brain cell death caused by ischemic stroke: preventing, delaying, or rescuing cells from death until the blood supply to the brain could be restored would greatly reduce, if not eliminate, the possibility of a person's death and/or long-term disability from stroke (Lee J M, et al. Nature 1999, 399(supp): A7-A14; Tarkowski E, et al. Stroke 1999, 30(2): 321-7; Pulera M R, et al. Stroke 1998, 29(12): 2622-30). In still other circumstances, the failure of cells to die may itself lead to disease such as cancer (Hetts S W. JAMA 1998, 297(4): 300-7).
Cell death plays an important role in the normal function of mammalian organisms. While it may seem counterintuitive for cells to have death as a normal part of their life cycle, controlled and physiologically appropriate cell death is important in regulating both the absolute and relative numbers of cells of a specific type. (Hetts S W. JAMA 1998, 297(4): 300-7; Garcia I, et al. Science 1992, 258(5080): 302-4). When the mechanism of apoptosis does not function properly and normal cell death does not occur, the resulting disease is characterized by unregulated cellular proliferation, as occurs in a neoplastic disease or an autoimmune disease (Hetts S W. JAMA 1998, 297(4): 300-7; Yachida M, et al. Clin Exp Immunol 1999, 116(1): 140-5).
One method for regulating cell death involves manipulating the threshold at which the process of cell death begins. This threshold varies significantly by cell type, tissue type, the type of injury or insult suffered by the cell, cellular maturity, and the physiologic conditions in the cell's environment (Steller H., Science 1995, 267(5203): 1445-9). Although it is probable that certain cellular injuries or insults irrevocably induce death, lesser injuries or insults may begin the dying process without inducing irreversible cell death. What constitutes a lesser injury or insult may vary tremendously with changes in the factors influencing that cell's death threshold. The ability to alter a cell's threshold for responding to an injury or insult, that is, to either promote or discourage cell death, would be a desirable goal for the treatment of conditions involving cell death. The ability to better control cell death, by either discouraging or promoting the mechanisms of cell death, would be an important invention for ameliorating disease (U.S. Pat. Nos. 5,925,640; 5,786,173; 5,858,715; 5,856,171).
Recent evidence suggests that the mechanisms of cellular death may be more complex than the two discrete pathways of apoptosis and necrosis. Examples of this evidence may be found in the central nervous system (CNS). In the complex CNS cellular environment, both necrosis and apoptosis are observed with commonly studied conditions, disorders, or diseases such as focal ischemia, global ischemia, toxic insults, prolonged seizures, excitotoxicity, and traumatic brain injury. In some reports, both apoptosis and necrosis have been described (Choi W S, et al. J Neurosci Res 1999, 57(1): 86-94; Li Y, et al. J Neurol Sci 1998, 156(2): 119-32; Lee J-M, et al. Nature 1999, 399(supp): A8-A14; Baumgartner W A, et al. Ann Thorac Surg 1999, 67(6): 1871-3; Fujikawa D G, et al. Eur J Neurosci 1999, 11(5): 1605-14; Gwag B J, et al Neuroscience 1999, 90(4): 1339-48; Mitchell I J, et al. 1998, 84(2): 489-501; Nakashima K, et al. J Neurotrauma 1999, 16(2): 143-51; Ginsburg, Md. Cerebrovascular Disease: Pathophysiology, Diagnosis, and Management 1998 Ch 42; Rink A D, et al. Soc Neurosci Abstr 1994, 20:250(Abstract)). Similar observations also occurred with brain tumor cells. (Maurer B J, et al. J Natl Cancer Inst 1999, 91(13): 1138-46) Other investigators found that neurons die by either apoptosis or necrosis under different environmental conditions (Taylor D L, et al. Brain Pathol 1999, 9(1): 93-117). There also are reports of a unique type of neuronal cell death following stroke. This new type of cell death has features common to both necrosis and apoptosis (Fukuda T, et al. Neurosci Res 1999, 33(1): 49-55). Other investigators believe that neuronal cell death is best represented by a continuum between apoptosis and necrosis, possibly mediated by calcium levels (Lee J-M, et al. 1999, 399(supp): A7-A14), or a combination of direct ischemic damage followed by indirect damage from excitotoxicity and loss of intemeuronal connections (Martin L J, et al. Brian Res Bull 1998, 46(4): 281-309). Further complicating the picture of neuronal cell death is the observation that the death of one or more neurons in one region of the brain can induce the death of neurons in other brain regions. This phenomenon has been observed with stroke as described above (Martin L J, et al. Brain Res Bull 1998, 46(4): 281-309) as well as neuronal cell death induced by the withdrawal of growth factors (Ryu B R, et al. J Neurobiol 1999, 39(4): 536-46). Given the complex nature of actions and interactions among the many physiologic and molecular forces in brain tissue, and the different abilities of many substances acting either alone or in combination to induce cellular injury or death, it is difficult to determine with any degree of certainty if a nerve cell death process is due to apoptosis or necrosis (Graham D I, Greenfield's Neuropathology Ch 3 1997).
Despite the challenges in classifying the mechanism of cellular death, there is broad agreement that most, if not all, cells share common features in their death mechanisms (see, e.g., Lee J. M., et al., Nature 1999, 399 (supp): A7-A14).